Don King -- I Love Justin Bieber ... He's Great for Boxing

Someone call a doctor ... 'cause Don King has Bieber Fever!!!

The 82-year-old boxing promoter extraordinaire was at USC yesterday to get ready for Bermane Stiverne vs. Chris Arreola ... when we asked how he felt about Floyd Mayweather having Justin Bieber and Lil Wayne in his corner during recent fights.

It's clear ... King's a HUGE fan -- telling TMZ Sports both guys are GREAT because they help bring "the masses" to the sport.

"I love 'em both! They're fabulous! They're Americana!"